I've never flushed the fluid, bought this 2014 9k miles ago at 132k, and knowing MOST owners, they never flush em, but I'm right there with ya, I know it absorbs water and will literally steam under hard braking. I've had experience with wet brake fluid in a motorcycle, and it caused a crash that really hurt
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!! I should flush the brake fluid TONIGHT, never mind I need brakes sometime in the next 30 days....
Reverting to some kind of electric drum brakes would be very impractical and I don't think it would help. I would describe my disc surge brakes as 'very good to outstanding', and when I brake hard I can feel the trailer slowing me down, no question they're working. It's a boat trailer, and I remember about 35 years ago my boat trailer had drum brakes and they wouldn't work for quite some time after being wet, pulling the boat out of the water AAND you had to manually adjust them because they lock out when reversing so auto-adjusters don't work. PITA. My last 9 boats all had disc surge brakes that worked great when maintained. The clunking when moving or stopping is an indication that A. your shock absorbing piston inside the surge assembly needs replacement or B. your surge brakes need bleeding.
You're right, downshifting is the only option on a loooong downhill, however, can our 6R80's handle that? My last truck was a Duramax with Allison 1000 and it braked downhill on it's own incredibly well, with no exhaust brake. This is my first 6R80.